Christian Henderson's volume on The Use of Force and International Law is a comprehensive exposition of the international legal regime governing recourse to force—jus ad bellum. Henderson, who is a professor of international law at the University of Sussex and co-editor of the Journal on the Use of Force and International Law, demonstrates comprehensive mastery of the subject throughout. The Use of Force and International Law, in the tradition of distinguished volumes like Ian Brownlie's International Law and the Use of Force by States, Yoram Dinstein's War, Aggression and Self-Defence, and Christine Gray's International Law and the Use of Force, embraces an “objective positivist” (p. 4) approach in an effort to describe and explicate the substantive content—the lex lata—of the law on the use of force. The volume does not present Henderson's preferred view of the law or advance a normative call for the development of jus ad bellum in a particular direction. That said, the law governing the use of force is characterized by many contentious claims and disagreements, which makes it a challenge to simply say what the law is. Henderson's book presents the competing views on many of these issues in a thoughtful, balanced, and scholarly manner, which makes the book highly suitable for use in teaching an advanced level course. As a whole, this is a deeply researched, comprehensive, and erudite contribution to the literature on the law governing recourse to force.

The Use of Force and International Law is divided into four main parts. Part I deals with threshold issues concerning the prohibition on the use of force in international law. Henderson does not devote a great deal of attention to the law on the use of force prevailing prior to 1945, but focuses largely on the legal regime adopted in the UN Charter. He carefully analyzes the scope and meaning of the prohibition on the use of force in Article 2(4) of the Charter, including engaging with the foundational question of what constitutes a “use of force,” and whether it covers all forms of political or economic coercion, or whether it is instead limited to uses of armed force. This discussion allows Henderson to address such issues as the legal status of non-consensual military incursions by one state's force into the territory of another state in which weapons are not actually used, i.e., where no shots are fired; the indirect use of force in which a state assists others (including nonstate actors) in perpetrating coercive acts; and the status of cyber operations. Given the comprehensive nature of the book and Henderson's sophistication as an analyst, it is unfortunate that his treatment of the status of cyber operations under the use of force regime is relatively brief. He endorses an effects-based view to analyze the legal character of cyber operations and suggests that such operations qualify as a use of force only if they “produce physical damage” (p. 59). Yet he does not address issues that have generated debate regarding the status of cyber operations, including those that “do not result in injury, death, damage, or destruction, but that otherwise have extensive negative effects,”Footnote ¹ such as an operation that temporarily disables but does not damage an adversary's defensive systems or an action against a stock exchange that causes the market to crash with severe economic ramifications.

In Part II, Henderson takes up the use of force “in the context of collective security,” i.e., where authorized by the UN Security Council pursuant to its Chapter VII authorities or, more controversially, when undertaken by regional organizations. Henderson reminds us, in “managing expectations of” the Security Council, that it “was envisaged as a body to enforce the peace, not as a body to enforce the law” (p. 90). Nevertheless, Henderson recounts that despite this limited mandate, the Council has, at least since the end of the Cold War, “taken a rather liberal approach” (p. 95) in interpreting the scope of its authority to address any “threat to the peace, breach of the peace, or act of aggression” in order to take action in connection with domestic crises. Henderson thus recognizes both the structural limits on the Security Council's ability to enforce international law and its efforts to expand those limits. Part II also includes a rich discussion of the evolution of the Security Council's “authorisation method” of conducting collective security operations, in which the Council authorizes coalitions of member states to conduct military operations to achieve its goals, including in Iraq (1990), Somalia (1992), Haiti (1994), Côte d'Ivoire (2011), and Libya (2011). Reliance on the authorization method, of course, was necessitated by the failure of states to conclude the “special agreements” contemplated under Article 43 of the Charter to make their armed forces available to the Security Council for the purposes of maintaining international peace and security. Although some have decried this as resulting in the “privatization” of the collective security function,Footnote ² Henderson notes that the contrast in operational control between the original Charter model and the status quo might not be as pronounced as is sometimes assumed. He observes that even under the scheme set out in Article 43 of the Charter that provided for “troops to be at the disposal of the Council, the actual control of these forces was not to fall to the Security Council” (p. 112). He concludes that Chapter VII, even as envisaged, “does not … provide for the Security Council to have any more command over the forces at its disposal than as has transpired under the authorisation mechanism,” at least at the tactical level (id.).

As a matter of “overall strategic control,” however, the authorization method has in some cases generated controversy about the aims and scope of military actions taken by “coalitions of the willing,” once the Council has authorized them to act pursuant to Chapter VII. Henderson analyzes the actions of NATO-led forces authorized in 2011 to use force “to protect civilians” against governmental forces in Libya, including support for opposition forces that ultimately contributed to the overthrow of the Gaddafi regime, and the efforts of French and UN Operations in Côte d'Ivoire (UNOCI) forces in forcibly removing Laurant Gbagbo from power. Henderson does not, in the end, take a position on whether the military forces in these cases exceeded the authority granted by the Security Council. Rather, in language that is characteristic of Henderson's non-dogmatic approach to the issues he covers, he concludes that “the interventions in both Libya and Côte d'Ivoire demonstrate the uncertainties that may exist and the margin of appreciation that those implementing the mandates arguably possess” (p. 143).

Like others who write about collective security,Footnote ³ Henderson discusses United Nations peacekeeping operations in addition to Security Council authorizations to use force. Instead of merely treading on familiar issues related to the legal foundations for peacekeeping operations, Henderson includes a detailed and interesting discussion of evolution of the legal basis for peacekeepers themselves to use force. He chronicles how the early limited understanding that peacekeepers could use force only in personal self-defense “widened to include resistance by forceful means to attempts to prevent the peacekeeping operation from implementing the mandate” given to it by the Security Council, or “defence of the mission or mandate” (pp. 190–91). Still other UN peacekeeping missions have been given “robust mandates to protect civilians under imminent threat of physical attack” (p. 191). Armed with these “ever more robust mandates” (p. 200), a number of UN peacekeeping forces have become actively engaged in fighting. Henderson's reminder that much of the actual use of force that takes place in the collective security realm arises not only in the context of the familiar Chapter VII peace enforcement actions (e.g., Korea (1950), Iraq (1990), Somalia (1992), and Libya (2011)), but also in the context of peacekeeping operations deployed in the midst of ongoing civil wars (e.g., UNPROFOR in the former Yugoslavia (1992–1995), UNOSOM II in Somalia (1993–1995), UNAMSIL in Sierra Leone (1999–2005), MONUC and MONUSCO in the Democratic Republic of the Congo (since 2000), and MINUSMA in Mali (since 2013)).

Part III turns to the right of states to use force in individual or collective self-defense under Article 51 of the Charter and customary international law. Here, Henderson grapples with some of the most contentious use of force debates. He carefully examines such questions as whether there is, as the International Court of Justice indicated in the Nicaragua Footnote ⁴ case, a “gap” between violence that constitutes a prohibited use of force under Article 2(4) of the Charter and more grave violence that amounts to an armed attack under Article 51, i.e., whether states are prohibited from invoking the right of self-defense to respond to relatively low-level uses of force; whether the customary international law right to use force to protect or rescue nationals has survived the adoption of the UN Charter; and procedural aspects of a state's ability to exercise collective self-defense on behalf of another state that is the victim of an armed attack. The two self-defense questions that draw the most attention from Henderson are those that arise in face of contemporary security threats that differ markedly from those the Charter was designed to address, namely, responding to violence perpetrated by nonstate actors and anticipatory uses of force aimed at forestalling a potentially devastating attacks, particularly those employing weapons of mass destruction (WMD).

On the question of the use of force against nonstate actors, Henderson surveys the competing positions that have engendered disagreement among different international law authorities. Although he is careful about staking out a definitive position, Henderson states that “it is an emerging view that nonstate actors are not excluded as the possible perpetrators of an armed attack” and “are arguably within the purview of the right of self-defence” (p. 308). This is a position that goes beyond the one that former State Department Legal Adviser John Bellinger reportedly encountered among “some Europeans,” who did not accept the notion that the United States was in a state of armed conflict with Al Qaeda and the Taliban in Afghanistan (and elsewhere) after September 11, 2001 “as part of a lawful action in self-defense against an armed attack.”Footnote ⁵ Even if states have a right of self-defense against nonstate actors, however, this does not address the question of the territorial integrity rights of the state where nonstate actors are located, but which may not be legally responsible for their actions. Henderson reviews theories that have gained prominence—or at least attention—in recent years as a basis for using force in the territory of another state in the exercise of the right of self-defense against nonstate actors located there, including arguments for lowering the standard of control needed to establish the state's legal responsibility for the conduct of nonstate actors and the “unwilling and unable” test championed by the United States.Footnote ⁶ Henderson concludes that “there remain significant question marks over the international legal status of the [unwilling or unable] doctrine,” and that “the precise parameters of the right to respond to an armed attack by [nonstate actors] with a use of force in foreign territory are still being worked out in practice” (pp. 332–33).

One surprising aspect of Henderson's discussion of the use of force, in the context of responses to attacks by both states and nonstate actors, is his apparent view that after a state has suffered an armed attack, it may use force only if it can establish that each responsive use of force meets the test of necessity. In support of this position, Henderson admittedly finds support in the International Court of Justice's judgment in the Oil Platforms Footnote ⁷ case, where the court questioned whether the U.S. attacks on Iranian oil platforms in the Persian Gulf were “a necessary act” in view of the Iranian actions to which the United States claimed to be responding (p. 231). This perspective seems to accord the traditional customary international law requirement of necessity an outsized role in a post-Charter world. Prior to the adoption of the Charter, when states were free to use force to enforce their rights by way of self-help even in the case of non-forcible breaches of international law, it made sense to require states to exhaust all peaceful means to resolve a dispute before resorting to force, i.e., to ensure that the use of force was necessary. Once an adversary has committed an armed attack against a state, however, it seems presumptively reasonable for that state to conclude that the controversy is no longer susceptible to resolution via peaceful means. Put another way, once a state of armed conflict exists, it seems to me, a state need not subject each prospective attack to a separate necessity test.

Henderson similarly discounts the possibility that a continuing sequence of attacks might be taken against nonstate actors once a state of non-international armed conflict with an armed group exists. He misapprehends the law, in my view, when he suggests that there is “need for a separate self-defence assessment to be undertaken on each occasion” (p. 342) when a state engaged in armed conflict against a nonstate armed group conducts operations against enemy fighters for that group.Footnote ⁸ There is, to be sure, much that is deeply problematic about the way the United States has applied the law governing the use of force in its military campaign against nonstate armed groups since 9/11; I am among those who have expressed criticism of aspects of the U.S. government's legal reasoning.Footnote ⁹ But once a state finds itself in a state of armed conflict, international law does not require it to meet the requirements of necessity prior to each armed operation or to make a de novo self-defense determination with respect to each military action against combatants or fighters for the other side.

In the self-defense portion of the book, Henderson also devotes attention to the claims about the right to use force prior to an armed attack, including both what he refers to as “anticipatory” self-defense (the use of force in the face of an imminent threat of armed attack) and “pre-emptive” self-defense (the use of force to head off more temporally remote threats). He suggests that the status of anticipatory self-defense under international law is ambiguous, but is decisively dismissive of preemptive self-defense. Indeed, the normally balanced and circumspect Henderson allows himself some editorial license in commenting on the preemption doctrine elaborated in the Bush Administration's 2002 National Security Strategy (NSS); he characterizes the NSS as an “an extraordinary document” that represented a “sharp break” from prevailing interpretations (p. 287); he refers to the preemption doctrine as one that was “peddled by President Bush after the attacks of 9/11” (p. 409).

In Part IV, Henderson addresses internal conflicts, either interventions (or counter-interventions) in the case of civil wars or humanitarian interventions to halt the commission of atrocities by a government against its own population. Given the positivist approach that characterizes most of the volume, Henderson makes a surprising claim that if one side in a civil war receives external support, other states have the right to counter-intervene, even on behalf of opposition groups, to preserve equality. This view suggests that foreign intervention on behalf of the Taliban, in response to U.S. support for the government of Afghanistan, or foreign intervention on behalf of Syrian opposition groups, in response to Russian support for the government of Syria, would be permissible. That seems highly doubtful in light of international law's strong opposition to intervention and its reluctance to “de-recognize” existing de jure governments until a new government has been formed to take its place. On humanitarian intervention, Henderson concludes quite firmly that there is no such right absent Security Council authorization, but notes the rise of normative arguments in favor of such a doctrine (p. 406).

Although Henderson states that his project is to describe what the law is, the most interesting sections of the book understandably embrace those issues as to which disagreements about the content of the law abound. Henderson, as previously noted, is generally balanced and cautious. He generally presents the arguments and evidence for different perspectives on contentious issues and tends not to adjudicate the differences. For instance, he espouses no final position on whether the 2011 interventions in Libya and Côte d'Ivoire went beyond the scope of the Security Council's authorization (pp. 142–43); on whether subsequent action by the Security Council retroactively authorized the interventions in Sierra Leone and Liberia by ECOMOG, a military force established by the Economic Community of West African States (pp. 153–54); and on the lawfulness of the use of force against nonstate armed groups under the “unwilling or unable” test (pp. 332–33). But that is not always so, and the book is not free of normative judgments. In some places, Henderson is willing to stake out stronger claims. For example, he effectively delivers “verdicts” rejecting the U.S. and UK argument that the 2003 invasion of Iraq was authorized by Security Council resolutions adopted at the time of the first Persian Gulf war (p. 134); the legality of the 1999 NATO use of force against Serbia to halt human rights violations in Kosovo (p. 147); the right of states to exercise anticipatory self-defense (at least as of 2002) (p. 287); Russia's claimed right to invade Crimea to protect Russian nationals (p. 256); and, as noted, the doctrine of humanitarian intervention (pp. 402, 406).

While I generally share Henderson's assessment on the issues noted in the preceding paragraph (except I think the understanding of the Charter had evolved, even prior to 2002, to recognize a right of anticipatory self-defense), more interesting than the question of any conclusion Henderson draws is the question of the method by which he reaches that conclusion. In other words, how does Henderson—and how to do we—evaluate what the law governing the use of force is? Early in the book, Henderson sets out a high bar for “normative change” of the black letter law governing the use of force—one that is “onerous and not easily met” (p. 26). Changing the law requires either “extensive state practice” or “unambiguous and widespread acceptance and recognition, perhaps in the form of a declaratory resolution adopted by consensus within the UN General Assembly” (id.).

Setting aside for a moment whether the test for transformation of the law on the use of force is exacting or flexible, it is not always clear what kind and degree of evidence Henderson believes is required to support a claim about either the proper interpretation of the law or claims that the law has changed. In some cases, Henderson's interpretive approach to a contested legal question is largely conceptual, reflecting normative judgments about how the law ought to operate. On the question of the permissibility of using force in self-defense against nonstate groups, for instance, Henderson identifies a series of factors—undoubtedly sensible ones—that would influence a judgment about whether a use of force against a nonstate group qualifies as self-defense (p. 246), but these factors do not seem to be deeply grounded in state practice or declarations. Similarly, Henderson's view that anticipatory self-defense (in the face of an imminent threat) is perhaps today reconcilable with Article 51's requirement of the existence of an armed attack, but that preemptive self-defense (against a remote threat) is not, also seems to be grounded in normative policy views rather than state practice or General Assembly declarations.

More frequently, Henderson examines disputes about the content of the law by considering selected cases in which states have used force on the basis of, or at least in a manner consistent with, a claimed legal principle and then assessing the reaction of states and commentators to the lawfulness of those operations. For example, in examining the threshold of violence required to meet the threshold of an “armed attack” within the meaning of Article 51, Henderson chronicles “a handful of examples” (p. 223). On the question of the right of states to use force in defense of their nationals, Henderson recounts a number of cases and assesses the international reaction—mainly comments made by states in the Security Council or the General Assembly (pp. 249–56). He notes that the U.S. invasion of Afghanistan after 9/11 “received general support within the international community” (p. 315) and that there was “little condemnation” of U.S. attacks against Sudan and Afghanistan following the 1998 East African embassy bombings (p. 319). On the issue of humanitarian intervention, Henderson suggests that international reaction to the cases from the 1970s sometimes cited as evidence in support of a right of humanitarian intervention (such as India's intervention in East Pakistan, Vietnam's intervention in Cambodia, and Tanzania's intervention in Uganda) “was, on the whole, negative” (p. 388).

To be clear, I endorse Henderson's focus on state practice, as well as reactions of states to the practice of other states, in seeking to ascertain the content of the law governing the use of force.Footnote ¹⁰ The approach follows in the distinguished tradition of Thomas FranckFootnote ¹¹ and Michael Reisman.Footnote ¹² Two points stand out, however. First, it is important to recall how much authority a method that involves interpreting the reactions of the international community confers upon the interpreter. In the context of a claimed right of anticipatory self-defense, for instance, Henderson interprets the fact that Israel's use of force during the 1967 Six-Day War was “not widely condemned” as reflecting a recognition of at least a narrow right of anticipatory self-defense, in limited circumstances (p. 281). But he does not interpret a similar lack of condemnation of Israel's 2007 strike against a nuclear reactor in Syria as an acceptance of a right of preemptive self-defense to counter more remote threats. Henderson concludes that “while states do not wish to see pre-emptive self-defence take its place lex lata within the law of self-defense, there may be times when they wish to say nothing” (p. 291). Yet it may be that the difference between these two cases lies more in the eye of Henderson as beholder than in the eyes of the states who reacted—or did not—to Israel's military actions.

Second, the reliance on relatively small numbers of cases and limited responses from states in shaping our understanding of the law governing armed conflict suggests that the standard for development and transformation of the law is not as exacting as Henderson initially suggests; a practice probably does not require the support of “virtually every state” (p. 25) to reflect an accepted interpretation of the UN Charter or to bring about normative change. Henderson himself at times seems to embrace this view this; he cites instances—such as the emergence of what has been referred to as “Chapter VI and a Half” UN peacekeeping operations—in which “the institution has evolved through the practice of the UN” (p. 172). Elsewhere, he states more generally that the Charter “is a living instrument” that “evolves and develops” (p. 121). Given Henderson's deep knowledge of this field of law, I would welcome a more systematic account of his view about how, in Franck's words, the Charter is susceptible of “continuous interpretation and adaptation.”Footnote ¹³ Henderson insightfully observes that “while the law is not, and should not be, infinitely malleable[,] … it does need to maintain its relevancy and effectiveness” (p. 346). Experts such as Henderson can help us to determine what evidence of institutional and state practice is necessary to strike the right balance.

On balance, Henderson's comprehensive, balanced, and scholarly treatise makes an excellent contribution to the literature on the law governing the use of force, one that brings the doctrine into lively discourse with contemporary events and cases. It will be a welcome resource for scholars and students alike.